Counter-rotating spindle for friction stir welding

ABSTRACT

A counter-rotating spindle includes a shoulder tool, a pin tool inserted in the shoulder tool, a first motor that is connected with the pin tool and rotates the pin tool, and a second motor that is connected with the shoulder tool and rotates the shoulder tool. The direction and the speed of the rotation for the pin tool may be selected independently from the speed and direction of rotation of the shoulder tool enabling optimization of the friction stir welding process. Counter-rotating the pin tool and the shoulder tool may improve the mixing abilities and efficiency of the pin tool and may prevent the galling effect between the pin tool and the shoulder tool. The counter-rotating spindle may be used, for example, for continuous path friction stir welding and spot welding of aluminum and its alloys, including cast alloys, as well as joining similar and dissimilar alloys.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/053,630, now U.S. Pat. No. 7,703,654, entitled “Counter-RotatingSpindle for Friction Stir Welding,” filed Feb. 7, 2005, which alsoclaims the benefit of U.S. Provisional Application No. 60/628,832, filedNov. 17, 2004, each of which are incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to friction stir welding toolsand welding processes, more particularly, to a counter-rotating spindlefor friction stir welding, to a continuous path friction stir weldingprocess, and to a process for spot welding.

Welding may be the most common way of permanently joining metal parts.Friction stir welding (FSW) was introduced in late 1991 by the TWIWelding institute, U.K. (U.S. Pat. No. 5,460,317). In friction stirwelding, a cylindrical, shouldered tool with a profiled pin is rotatedand slowly plunged onto the work-pieces of sheet or plate material,which are lapped or butted together. The work-pieces have to be clampedrigidly onto a backing bar or engineered substructure in a manner thatprevents the abutting joint faces from being forced apart. Frictionalheat is typically generated between the wear resistant welding tool andthe material of the work-pieces. This heat, along with the heatgenerated by the mechanical mixing process and the adiabatic heat withinthe material, causes the stirred materials to soften without reachingthe melting point, allowing the traversing of the welding tool along theweld line. The plasticized material is transferred from the leading edgeof the tool to the trailing edge of the tool pin and is forged by theintimate contact of the tool shoulder and the pin profile. The weldingof the material is facilitated by severe plastic deformation in thesolid state involving dynamic recrystallization of the base material. Asolid phase bond between the two work-pieces is created. The frictionstir welding process can be regarded as a solid phase keyhole weldingtechnique since a hole to accommodate the pin of the tool is generatedand then filled during the welding sequence. Since the friction stirwelding process takes place in the solid phase below the melting pointof the materials to be joined, materials that are difficult to weldusing traditional fusion welding methods can now be joined, for example,2000 and 700 aluminum alloys. Other advantages include, for example,excellent mechanical properties of the weld, usage of a non-consumabletool, and elimination of the need for consumable welding products. Thedescribed friction stir welding process using a tool with a fixed pinlength has the disadvantages of only being able to join work-pieceshaving the same thickness, and of leaving a keyhole in the work-piecewhen the pin is retracted at the end of the weld line. This hole must becovered, for example, with a rivet in order to preserve the integrity ofthe weld. Furthermore, the friction stir welding process is usually usedto produce straight-line continuous path welds. Due to the forming ofthe keyhole when retracting the tool, friction stir welding is typicallynot used for spot welding. The friction stir welding process can be usedfor the manufacture of many joint geometries, such as butt welds,overlap welds, T-sections, fillet, and corner welds. For each of thesejoint geometries specific tool designs are required.

An auto-adjustable pin tool for friction stir welding was developed byengineers at Marshall Space Flight Center of the National Aeronauticsand Space Administration (NASA) (U.S. Pat. No. 5,893,507). Theretractable pin tool (RPT) uses a computer-controlled motor toincrementally retract the pin into the shoulder of the tool away fromthe work-piece at the end of the weld preventing keyholes. The design ofthe tool allows the pin angle and length to automatically adjust tochanges in material thickness and results in a smooth hole closure atthe end of the weld. Current retractable pin tools utilize two separateparts, a shoulder and a pin. The pin is positioned within the shoulder.When welding the rotating pin enters the work-pieces to be welded andstirs the material while the shoulder impacts the surface of thework-pieces. Since there is a tolerance between the shoulder and thepin, hot material may be able to migrate up the shaft between shoulderand pin during the welding process. Once the migrated material cools itmay temporary and locally weld the pin to the shoulder. This gallingeffect may damage the surfaces of the pin and/or the shoulder. In allknown retractable pin tool applications the pin and shoulder rotate atthe same speed and in the same direction. In some applications,depending on the material to be welded, overheating or even meltingalong the work-piece surface close to the shoulder may occur that may becaused by the rotation of the shoulder. This can lead to undesirablefusion related defects of the work-piece surface.

As can be seen, there is a need for a friction stir welding tool thatmay be used for a variety of joint geometries, that eliminates thegalling effect between the shoulder and the pin during the weldingprocess, and that eliminates overheating of the work-piece surface closeto the shoulder of the tool. Furthermore, there is a need for a frictionstir welding tool that may be used to reduce the surface indent duringthe friction stir spot welding process. Moreover, there is a need for afriction stir welding process that produces welds with improvedsmoothness over the entire length and with improved durability of thejoints.

There has, therefore, arisen a need to provide a versatile friction stirwelding spindle. There has further arisen a need to provide a stirfriction welding spindle that enables an improved mixing of thematerials to be welded. There has still further arisen a need to providea process for friction stir welding producing continuous path welds thathave optimized mechanical properties and are smooth over the entirelength. There has still further arisen a need to improve the frictionstir spot welding process by reducing material loss, by reducing surfaceindent, and by eliminating weld nugget voids.

SUMMARY OF THE INVENTION

The present invention provides a counter-rotating spindle for frictionstir welding, a continuous path friction stir welding process, and aprocess for spot welding. The present invention enables the independentrotation of a separate shoulder tool and a retractable pin tool forfriction stir welding. The present invention provides a versatilefriction stir welding tool that is suitable for, but not limited to,applications in the aerospace industry, shipbuilding and marineindustries, railway industries, automobile industry, and constructionindustry. The counter-rotating spindle may be used, for example, forwelding aluminum and its alloys, including cast, wrought, and extrudedalloys, as well as joining similar and dissimilar alloys.

In one aspect of the present invention, a counter-rotating spindlecomprises a shoulder tool, a pin tool, a first motor connected with thepin tool, and a second motor connected with the shoulder tool. The pintool is inserted in the shoulder tool. The pin tool is movable insidethe shoulder tool. The first motor rotates the pin tool. The secondmotor rotates the shoulder tool.

In another aspect of the present invention, a friction stir weldingspindle comprises a spindle housing, a nosepiece housing attached to thespindle housing, and a counter-rotating spindle inserted into thenosepiece housing. The counter-rotating spindle extends into the spindlehousing.

In a further aspect of the present invention, a friction stir weldingprocess comprises the steps of: rotating a pin tool at a first constantspeed in a first direction, rotating a shoulder tool at a secondconstant speed in a second direction, penetrating the work-pieces with apin of the pin tool until a shoulder of the shoulder tool contacts asurface of the work-pieces, generating frictional heat to plasticize thematerial of the work-pieces forming a continuous weld, and reversing thefirst direction of the rotation of the pin tool before retracting thepin from the material. The shoulder tool surrounds the pin tool.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a friction stir welding head accordingto one embodiment of the present invention;

FIG. 2 is a perspective view of a nosepiece assembly according to oneembodiment of the present invention;

FIG. 3 is an exploded view of the nosepiece assembly illustrated in FIG.2 according to one embodiment of the present invention;

FIG. 4 is an exploded view of a nosepiece assembly for friction stirspot welding according to another embodiment of the present invention;

FIG. 5 is a flow chart of a friction stir welding process according toanother embodiment of the present invention; and

FIG. 6 is a flow chart of a friction stir spot welding process accordingto another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Broadly, an embodiment of the present invention provides acounter-rotating spindle for friction stir welding, a continuous pathfriction stir welding process, and a process for spot welding. Contraryto the prior art, the counter-rotating spindle as in one embodiment ofthe present invention enables the independent rotation of a shouldertool and a pin tool of a friction stir welding spindle. Consequently,the pin tool may be rotated in the same or the opposite direction as theshoulder tool and, furthermore, the pin tool may be rotated at the sameor a different speed as the shoulder tool. Currently, it is onlypossible to rotate the pin tool and the shoulder tool simultaneously inthe same direction at the same speed. An embodiment of the presentinvention provides a counter-rotating spindle for stir friction weldingthat is suitable for, but not limited to, continuous path friction stirwelding and spot welding. An embodiment of the present inventionprovides a counter-rotating spindle for stir friction welding that issuitable for, but not limited to, applications in the aerospaceindustry, shipbuilding and marine industries, railway industries,automobile industry, and construction industry. The counter-rotatingspindle, as in one embodiment of the present invention, may be used, forexample, to weld skins to spars, ribs, and stringers for use in militaryand civilian aircraft. It may further be possible to use thecounter-rotating spindle as in one embodiment of the present inventionfor producing, for example, longitudinal butt welds, lap welds, spotwelds, tapered butt welds, and 5-axis contour welds, preferably, but notlimited to, of aluminum alloys.

In one embodiment, the present invention provides a counter-rotatingspindle for friction stir welding that is designed as a coaxial spindleand includes a retractable pin tool and a separate shoulder tool. Thepin tool and the shoulder tool may be driven independently by separatemotors and electronic drives enabling counter-rotation at differentspeeds of the pin tool and the shoulder tool. The counter-rotation anddifferent speeds of the pin tool and the shoulder tool could prevent orat least limit a galling effect that occurs while using prior artretractable pin tools. Furthermore, driving the pin tool and theshoulder tool independently will allow a user to choose a lower speedfor the shoulder and, therefore, overheating or melting of thework-piece surface close to the shoulder, as is possible during theoperation of prior art friction stir welding tools, can be eliminated.Still further, driving the pin tool and the shoulder tool independently,as in one embodiment of the present invention, will enable optimizationof the welding parameter and, therefore, the material mixing ability aswell as the ability to mix dissimilar material may be improved comparedto prior art friction stir welding techniques.

In one embodiment, the present invention provides a continuous pathfriction stir welding process that produces a weld with mechanicalproperties that are improved compared to welds produced with prior artfriction stir welding processes. By rotating the pin tool faster thanthe shoulder tool during the friction stir welding process, thecrushing, stirring, and forging action of the pin tool and,consequently, the microstructure of the weld may be improved overexisting friction stir welding processes. Furthermore, rotating the pintool faster than the shoulder tool may improve the welding speed, makingthe friction stir welding process as in one embodiment of the presentinvention more effective than prior art friction stir welding processes.Still further, rotating the pin tool in a direction opposite to theshoulder tool, as in one embodiment of the present invention, mayfurther enhance the mentioned advantages of rotating the pin tool andthe shoulder tool at different speeds and, furthermore, the gallingeffect typical for prior art friction stir welding processes using aretractable pin tool, may be eliminated.

In one embodiment, the present invention provides a spot welding processthat may include the steps of rotating the pin tool in a first directionat a first speed during the penetration of the work-pieces to be weldedand during the softening of the material, and rotating the pin tool in asecond direction opposite to the first direction and at a second speedwhich may differ from the first speed during the retraction of the pintool. By using the spot welding process as in one embodiment of thepresent invention it may be possible to create a high quality weld withimproved mechanical properties compared with prior art friction stirwelding techniques. Furthermore, reversing the direction of the rotationof the pin tool while retracting the pin tool may force the materialback into the spot weld reducing the size of the spot weld surfaceindent or eliminating the spot weld surface indent and providing asmooth weld, which is not possible using prior art stir friction weldingprocesses.

Referring now to FIG. 1, a friction stir welding head 10 is illustratedaccording to one embodiment of the present invention. The friction stirwelding head 10 may include a c-axis yoke 11 and a friction stir weldingspindle 12. The c-axis yoke 11 may include two rails 15 and twotrunnions 16. The c-axis yoke 11 may be attached to the c-axis of anumerically controlled stir friction welding machine. The rails 15 maybe in a fixed connection with trunnions 16. The friction stir weldingspindle 12 may include a spindle housing 13 and a nosepiece housing 21.The nosepiece housing 21 may be attached to the spindle housing 13 suchthat the nosepiece housing 21 may be able to rotate. The spindle housing13 may be attached to the rails 15 such that spindle 12 may be axiallymoved along the rails 15 (quill-axis) and such that the spindle 12 maybe rotated around a trunnion-axis 17. The friction stir welding spindle12 may include a counter-rotating spindle 37 that may include twoseparate motors (22 and 26) to drive a pin tool 25 and a shoulder tool29 independently (as shown in FIGS. 2 and 3). The spindle 12 may be usedfor continuous path friction stir welding as well as spot welding.

Referring now to FIGS. 2 and 3, a nosepiece assembly 20 is illustratedin assembled configuration (FIG. 2) and in an exploded view (FIG. 3)according to one embodiment of the present invention. The nosepieceassembly 20 may be part of the friction stir welding spindle 12 shown inFIG. 1. The nosepiece assembly 20 may include a nosepiece housing 21, afirst motor 22, a first drive spindle 221, a first tool holder 23, afirst collet 24, a pin tool 25, a second motor 26, a second drivespindle 261, a second tool holder 27, a second collet 28, a shouldertool 29, and a union nut 31.

The pin tool 25 may have a cylindrical shape and may extendlongitudinally along an axis 32 from a front end 251 to a back end 252having an outer diameter 253. The pin tool 25 may include a pin 36located approximately to the front end 251. The pin 36 may be threaded.During the friction stir welding process, the pin tool 25 may bepreferably rotated such that the plasticized material is forced downwardand backward. The first collet 24 may grip the back end 252 of the pintool 25. The first collet 24 holding the pin tool 25 may be insertedinto the first tool holder 23. The first collet 24 may ensure that thepin tool 25 stays centered along axis 32. The first tool holder 23 mayconnect the pin tool 25 with the first drive spindle 221. The firstdrive spindle 221 may connect the pin tool 25 with the first motor 22.Consequently, the first motor 22 may be used to rotate the pin tool 25.

The shoulder tool 29 may have a cylindrical shape and may extendlongitudinally along the axis 32 from a front end 291 to a back end 292.The shoulder tool 29 may include a front section 33 and a back section34. The front section 33 may have a smaller outer diameter than the backsection 34 and a shoulder 35 may be formed where the front section 33meets the back section 34. The shoulder tool 29 may be hollow and mayhave an inner diameter 293 that is larger than the outer diameter 253 ofthe pin tool 25 such that the pin tool 25 may be inserted in theshoulder tool 29 and may be movable along the axis 32 inside of theshoulder tool 29. The pin 36 may extend the shoulder tool 29 at thefront end 291. The second collet 28 may grip the back end 292 of theshoulder tool 29. The second collet 28 holding the shoulder tool 29 maybe inserted into the second tool holder 27. The second collet 28 mayensure that the shoulder tool 29 stays centered along axis 32. Thesecond tool holder 27 may connect the shoulder tool 29 with the seconddrive spindle 261. The union nut 31 secures the second tool holder 27 tothe second drive spindle 261. The second drive spindle 261 may connectthe shoulder tool 29 with the second motor 26. Consequently, the secondmotor 26 may be used to rotate the shoulder tool 29. The shoulder tool29, the second collet 28, the second tool holder 27, the second drivespindle 261, and the second motor 26 are hollow and extendlongitudinally along axis 32 when assembled. The pin tool 25, the firstcollet 24, the first tool holder 23, the first drive spindle 221, andthe first motor 22 may be assembled to form a first spindle extendinglongitudinally along the axis 32. The shoulder tool 29, the secondcollet 28, the second tool holder 27, the second drive spindle 261, theunion nut 31, and the second motor 26 may be assembled to form a secondspindle extending longitudinally along the axis 32. The first spindlemay be inserted into the second spindle to form a counter-rotatingspindle 37 extending longitudinally along the axis 32, as shown in FIG.2. The counter-rotating spindle 37 may be designed as a coaxial spindle,as shown in FIGS. 2 and 3. The counter-rotating spindle 37 may beinserted into the nosepiece housing 21. The nosepiece housing 21 may beattached to the spindle housing 13 of the friction stir welding spindle12, as shown in FIG. 1. The independent drive of the pin tool 25 and theshoulder tool 29 may also be accomplished by using a block type spindleincluding the first motor 22 and the second motor 26 that power the pintool 25 and the shoulder tool 29, respectively, through a belt drivemechanism (not illustrated). The first motor 22 and the second motor 26may be independently controlled by a computer driven numericalcontroller.

Due to the coaxial design of the counter-rotating spindle 37, the pintool 25 is movable along axis 32 and, thus, the pin tool 25 may beoperated as a retractable pin tool. The pin tool 25 may automaticallyadjust to changes in the thickness of the work-pieces to be welded andmay be retracted from the work-piece at the end of the welding processincrementally and while the shoulder tool 29 maintains contact with thesurface of the work-pieces. By connecting the pin tool 25 with a firstmotor 22 and by connecting the shoulder tool 29 with a second motor 26,the pin tool 25 and the shoulder tool 29 may be driven independently.Consequently, it may be possible to rotate the pin tool 25 and theshoulder tool 29 in opposite directions. It may further be possible torotate the pin tool 25 and the shoulder tool 29 at different speeds. Forexample, rotating the pin tool 25 at a higher speed than the shouldertool 29 may improve the mixing ability of the pin tool while reducingthe risk of overheating of the work-piece surface close to the shoulder35 of the shoulder tool 29. Rotating the pin tool 25 and the shouldertool 29 in opposite directions may further improve the crushing,stirring, and forging action of the pin tool 25 which may result in amore effective mixing, for example, of dissimilar materials, resultingin a high quality weld with desirable mechanical properties.Furthermore, counter-rotation of the pin tool 25 and the shoulder tool29 may reduce or eliminate galling the pin tool 25 to the shoulder tool29, which may be caused by hot material migrating into the shaft betweenthe pin tool 25 and the shoulder tool 29 during the friction stirwelding process.

Referring now to FIG. 4, a nosepiece assembly 30 for friction stir spotwelding is illustrated according to another embodiment of the presentinvention. The nosepiece assembly 30 may be part of the friction stirwelding spindle 12 (shown in FIG. 1) and may be used for friction stirspot welding instead of the nosepiece assembly 20 (FIGS. 2 and 3). Thenosepiece assembly 30 may include all elements of the nosepiece assembly20 (a nosepiece housing 21, a first motor 22, a first drive spindle 221,a first tool holder 23, a first collet 24, a pin tool 25, a second motor26, a second drive spindle 261, a second tool holder 27, a second collet28, a shoulder tool 29, and a union nut 31), as illustrated in FIGS. 2and 3 and as described above. The nosepiece assembly 30 may furtherinclude a pressure foot clamp assembly 38, as shown in FIG. 4. Thepressure foot clamp assembly 38 may include a non-rotating fixed housing381, attachment fasteners 382, a spring 383, a pressure foot clamp 384,an anti-roll pin 385, and a retaining cap 386. The non-rotating fixedhousing 381 may be attached to the nosepiece housing 21 using theattachment fasteners 382. The spring 383 may be inserted in an openingin the housing 381. The spring 383 may be compressed using the pressurefoot clamp 384. The retaining cap 386 may be attached, for example,screwed on to the housing 381 retaining the spring 383 and the pressurefoot clamp 384. The spring 383 may be a commercial available spring, forexample, a danly die spring #9-3214-36 with a 271 lbs./10 deflection.The function of the pressure foot clamp assembly 38 may be to captureand to retain the plasticized material from the hole caused by the pin36 while penetrating the two work pieces.

Referring now to FIG. 5, a continuous path friction stir welding process40 is illustrated according to another embodiment of the presentinvention. The process 40 may include the steps of: clamping twowork-pieces to be welded onto a backing bar or engineered substructurein a manner that prevents the abutting joint faces from being forcedapart (step 41) and forming a weld seam, rotating the pin tool 25 at afirst constant speed in a first direction (step 42), rotating theshoulder tool 29 at a second constant speed in a second direction (step43), penetrating the two work-pieces to be welded with the pin 36 of thepin tool 25 until the shoulder 35 of the shoulder tool 29 contacts thesurface of the working-pieces (step 44), and generating enoughfrictional heat between the shoulder 35 and the pin 36 to plasticize thematerial at the abutting joint faces of the two work-pieces (step 45).The shoulder 35 of the shoulder tool 29 may be used to apply pressure tothe surfaces of the two work-pieces to be welded. Furthermore, theshoulder 35 may prevent the plasticized material from leaving the weldline by forcing it back into the hole made by the pin 36 as the pin tool25 and the shoulder tool 29 traverses steadily and continuously alongthe weld seam in step 46. While the pin 36 traverses along the weld seamin step 46, the plasticized material is transferred from the leadingedge of the pin 36 to the trailing edge and while the pin tool 25 movesaway from the plasticized material, the material solidifies to become aductile, high strength, solid-state weld. The pin 36 needs to beretracted from the work-piece at the end of the weld line (step 49).Retracting the pin from the material (step 49) may include stopping therotation of the pin tool 25 (step 47), activating the rotation of thepin tool 25 in the opposite direction (step 48), and withdrawing the pintool 25 while the shoulder tool 29 still maintains contact with thesurface of the work-pieces (step 49). Furthermore, the rotation speed ofthe pin tool may be changed during the retracing step 49. By reversingthe direction of the rotation of the pin tool (step 48), the extractinghole may be minimized or eliminated and the end of the weld line mayhave a smooth finish. The pin tool 25 may be rotated at a differentspeed or at the same speed as the shoulder tool 29 (steps 42A, 42B, and42C). Rotating the pin tool 25 at a faster speed than the shoulder toolmay improve the path velocity as well as the mixing ability of the pintool 25. Counter-rotating the pin tool 25 and the shoulder tool 29 (step42D) may optimize the mixing abilities and the mixing effectiveness ofthe pin 36 as well as the ability of the shoulder 35 to force thematerial back into the welding hole. The result may be a smooth andstrong high quality weld. Furthermore, counter-rotating the pin tool 25and the shoulder tool 29 (step 42D) may prevent or at least minimize thegalling of the pin tool 25 to the shoulder tool 29 and may reduce thedowntime of the welding machine caused by galling. The method 40 stillallows the pin tool 25 and the shoulder tool 29 to be rotated in thesame direction (step 42E). The continuous path friction stir weldingprocess 40 may be used to produce, for example, butt joints, lab joints,“T” joints of tapered thickness, and 5-axis contour joints. The process40 may be used to friction stir weld, for example, aluminum alloys 2024,2124, and A357 aluminum cast material from 0.05 to 2.0 inch thickness(and variations in between). Counter-rotating the pin tool 25 and theshoulder tool 29 may also increase the mixing effectiveness, forexample, in dissimilar metals.

Referring now to FIG. 6, a friction stir spot welding process 50 isillustrated according to another embodiment of the present invention.The spot welding process 50 may include the steps of: clamping twowork-pieces to be welded onto a backing bar or engineered substructurein a manner that prevents the abutting joint faces from being forcedapart (step 51) and forming a weld seam, applying pressure with aclamping anvil to the work-pieces (step 52), rotating the pin tool 25 ata first speed in a first direction (step 53), rotating the shoulder tool29 at a second speed in a second direction (step 54), penetrating thetwo work-pieces to be welded with the pin tool 25 until the desireddepth is achieved (step 55), stopping the rotation of the pin tool 25(step 56), activating the rotation of the pin tool 25 in the directionopposite to the first direction (step 57), and withdrawing the pin 36from the work-pieces and placing shoulder tool 29 into force controlmode (step 58). The withdrawal of the pin 36 may be stopped in step 59at the point in which the front end 251 of the pin tool 25 is in thesame plane as the front end 291 of the shoulder tool 29. The shouldertool 29 may be removed from force control and placed into positioncontrol in step 61. It may now be possible to retract the pin tool 25and the shoulder tool 29 from the work-pieces together. During theretraction of the pin 36 from the material (step 58), the material maybe forced back into the extraction hole due to the reversed rotation ofthe pin tool 25 (step 57). The pin 36 may be retracted from the materialincrementally (steps 58 and 61). Furthermore, the rotation speed of thepin tool 25 may be changed before the retraction in step 58 or duringthe retraction in step 61. By reversing the direction of the rotation ofthe pin tool 25 during the spot welding process 50, a weld with improvedmechanical properties as well as a weld with a smooth surface may beobtained. The pin tool 25 may be rotated at a different speed or at thesame speed as the shoulder tool 29 (steps 53A, 536, and 53C). Rotatingthe pin tool 25 at a faster speed than the shoulder tool may improve themixing ability of the pin tool 25. Counter-rotating the pin tool 25 andthe shoulder tool 29 (step 53D) may optimize the mixing abilities andthe mixing effectiveness of the pin 36, for example, for dissimilarmaterials, as well as the ability of the shoulder 35 to force thematerial back into the welding hole. Furthermore, counter-rotating thepin tool 25 and the shoulder tool 29 (step 53D) may prevent or at leastminimize the galling of the pin tool 25 to the shoulder tool 29 and mayreduce the downtime of the welding machine caused by galling. The method50 still allows the pin tool 25 and the shoulder tool 29 to be rotatedin the same direction (step 53E), if needed. The process 50 may be usedto spot weld, for example, aluminum alloys 2024, 2124, and A357, forapplications, for example, in the aerospace industry.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. A friction stir welding process, comprising: rotating a pin tool at afirst constant speed in a first direction; rotating a shoulder tool at asecond constant speed in a second direction, wherein the shoulder toolsurrounds the pin tool; penetrating two work-pieces to be welded with apin of the pin tool until a shoulder of the shoulder tool contacts asurface of the work-pieces; generating frictional heat to plasticizematerial of the work-pieces forming a continuous weld; reversing thefirst direction of the rotation of the pin while maintaining rotation ofthe shoulder tool in the second direction; and after reversing the firstdirection of the rotation of the pin tool, retracting the pin from thematerial while the shoulder tool maintains contact with the surface ofthe work pieces such that the pin retracts vertically.
 2. The frictionstir welding process of claim 1, further comprising: clamping the twowork-pieces onto a backing bar forming a weld seam; and traversing thepin tool and the shoulder tool along the weld seam.
 3. The friction stirwelding process of claim 1, further comprising clamping the twowork-pieces onto an engineered substructure forming a weld seam.
 4. Thefriction stir welding process of claim 1, wherein reversing thedirection of the rotation of the pin tool comprises: stopping the firstrotation of the pin tool; and activating rotation of the pin tool in thedirection opposite to the first rotation.
 5. The friction stir weldingprocess of claim 1, further comprising selecting the first speed to befaster than the second speed.
 6. The friction stir welding process ofclaim 1, further comprising selecting the first speed to be equal to thesecond speed.
 7. The friction stir welding process of claim 1, furthercomprising selecting the first speed to be slower than the second speed.8. The friction stir welding process of claim 1, further comprisingselecting the first direction to be opposite to the second direction. 9.The friction stir welding process of claim 1, further comprisingselecting the first direction to be identical with the second direction.10. A friction stir welding process, comprising: rotating a pin tool ina first direction; rotating a shoulder tool in a second direction;penetrating two work-pieces with the pin tool until the shoulder toolcontacts a surface of the work-pieces; traversing the pin tool and theshoulder tool along a weld seam; after traversing the pin tool and theshoulder tool along the weld seam, reversing the first direction ofrotation of the pin tool while maintaining rotation of the shoulder toolin the second direction; and after reversing the first direction ofrotation of the pin tool, retracting the pin tool vertically from thetwo work-pieces.
 11. The friction stir welding process of claim 10,wherein retracting the pin tool vertically from the two work-piecescomprises retracting the pin tool vertically from the two work-pieceswhile the shoulder tool maintains contact with the surface of the twowork-pieces.
 12. A friction stir welding process, comprising: rotating apin tool at a first speed in a first direction; rotating a shoulder toolat a second speed in a second direction, wherein the shoulder toolsurrounds the pin tool; penetrating two work-pieces to be welded withthe pin tool until the shoulder tool contacts a surface of thework-pieces; reversing the first direction of the rotation of the pintool while maintaining rotation of the shoulder tool in the seconddirection; vertically retracting the pin tool from the two work-pieces;and during retraction of the pin tool, rotating the pin tool in adirection opposite the first direction at a third speed that isdifferent from the first speed.
 13. The friction stir welding process ofclaim 12, further comprising capturing and retaining plasticizedmaterial from a hole created in the work-pieces during penetration ofthe work-pieces by the pin tool utilizing a pressure foot clamp assemblyof a housing encompassing a portion of the shoulder tool and the pintool.